There is provided an oligomerization process. The process involves the use of a catalyst comprising an acidic solid. The acidic solid may comprise a Group IVB metal oxide, such as zirconia, modified with an oxyanion of a Group VIB metal, such as tungsten. The oligomers produced by this process may be hydrogenated to produce thermally stable lubricants and lubricant additives.
Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for a large number of years and have led to the introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants produced by the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wider range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants exhibit lower friction characteristics and are therefore capable of increasing mechanical efficiency of various types of equipment including engines, transmissions, worm gears and traction drives, doing so over a wider range of operating conditions than mineral oil lubricants.
PAO lubricants are often formulated with additives to enhance those properties for specific applications. Among the more commonly used additives are oxidation inhibitors, rust inhibitors, metal passivators, antiwear agents, extreme pressure additives, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like. This aspect of lubricant technology is described in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., 14, 477-526, to which reference is made for a description of the use of such additives.
PAOs useful as synthetic base stocks or functional fluids may be synthesized by homogeneous catalysts, such as promoted BF.sub.3 or AlCl.sub.3 catalysts. The synthesis of PAOs with a promoted BF.sub.3 catalyst is discussed in the Theriot et al. U.S. Pat. No. 5,171,905. The PAO processes using homogeneous catalysts always include a complicated and tedious catalyst separation step. For example, the promoted BF.sub.3 or AlCl.sub.3 catalyst is usually deactivated and destroyed by washing with sodium hydroxide, dilute acid and water consecutively. This separation step generates waste and is tedious. Therefore, it would be advantageous to use a solid and regenerable catalyst which can be separated easily from product and regenerated for reuse.